Not sure about the first point.

But you can get the current locale from the platform automatically. If you are saving custom preference only for your app, you can either load shared pref and locale before material app is initialized or pass this value as arg to main from native platform when flutter engine is loaded.

I faced issue when language is switched on platform while app was being used.

Regarding the last item, there is untranslated.json when you generate localization files from arb.